U.S. patent number 5,208,504 [Application Number 07/643,923] was granted by the patent office on 1993-05-04 for saw device and method of manufacture.
This patent grant is currently assigned to Raytheon Company. Invention is credited to James A. Greer, Thomas E. Parker.
United States Patent |
5,208,504 |
Parker , et al. |
May 4, 1993 |
Saw device and method of manufacture
Abstract
A SAW oscillator package having low vibration sensitivity
includes a stiffener layer disposed between a mounting surface for
the oscillator and the oscillator circuit package. Preferably, the
stiffening layer comprises a slab of a highly stiff material such
as a ceramic and, in particular, aluminum oxide, as well as other
material which preferably have suitable thermal expansion
coefficients matched between the material of the package and the
mounting surface. Moreover, the rigidly of the SAW device itself is
also increased by either reducing the lateral dimensions of the SAW
device or increasing the thickness of the SAW substrate and/or
cover.
Inventors: |
Parker; Thomas E. (Framingham,
MA), Greer; James A. (Andover, MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
24582718 |
Appl.
No.: |
07/643,923 |
Filed: |
December 28, 1990 |
Current U.S.
Class: |
310/313R;
310/344 |
Current CPC
Class: |
H03H
9/02818 (20130101); H03H 9/02984 (20130101); H03H
9/0542 (20130101); H03H 9/059 (20130101); H03H
9/1071 (20130101) |
Current International
Class: |
H03H
9/05 (20060101); H03H 9/02 (20060101); H01L
041/08 () |
Field of
Search: |
;310/313R,313A,313D,348,344,346 ;333/151,153,194,195,155
;331/135,176,65,17A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Bikash K. Sinha, Stanley Locke, "Acceleration and Vibration
Sensitivity of SAW Devices", IEEE Transations vol. 34, No. 1 Jan.
1987..
|
Primary Examiner: Budd; Mark O.
Attorney, Agent or Firm: Sharkansky; Richard M.
Claims
What is claimed is:
1. A SAW oscillator comprising:
means for providing a closed loop having an integral multiple of
2.pi. radians phase shift and excess small signal gain at a
frequency said means including a surface acoustic wave device
disposed to stabilize the frequency and phase characteristics of
said means;
a package comprising a base having a first stiffness characteristic
supporting said providing means, and
means disposed on said base for increasing the stiffness
characteristic of said base.
2. The oscillator, as recited in claim 1, wherein said stiffening
means includes a slab of material having a relatively high Young's
modulus compared to the corresponding Young's modulus
characteristic of the material of the package.
3. The oscillator, as recited in claim 1, where said means
comprises a slab of aluminum oxide.
4. The oscillator, as recited in claim 1, further comprising a
cover disposed over the oscillator package, said cover having a
second stiffness characteristic and said oscillator further
comprising second means disposed over said cover for increasing the
stiffness characteristic of said cover.
5. The oscillator, as recited in claim 4, wherein said second means
is a slab of aluminum oxide.
6. The oscillator recited in claim 1 wherein the base has a first
surface supporting the providing means and a second surface having
disposed thereon the stiffness increasing means.
7. A SAW oscillator comprising:
A SAW device comprising:
a substrate having a surface which supports surface wave
propagation and having coupled to said surface wave propagation a
pair of transducers;
a cover disposed to enclose said surface wave propagation surface
of said SAW device; and
a glass frit seal disposed between said cover and said
substrate;
means for providing a closed loop having an integral number of
2.pi. radians of phase shift and a excess small signal gain at a
frequency with said closed loop providing means including said SAW
device;
an oscillator package comprising:
a base supporting said SAW device, said base having a first
stiffness characteristic;
a cover disposed to enclose said base portion of the oscillator
package, said cover having a second stiffness characteristic;
means, disposed on said base portion of the oscillator package for
increasing the stiffness characteristic of said base; and
means, disposed on said cover portion of the oscillator package for
increasing the stiffness characteristic of said cover.
8. The SAW device, as recited in claim 7, wherein said means for
stiffening the cover is a slab of material having a stiffness
characteristic which is higher than the stiffness characteristic of
said cover portion of the oscillator package.
9. The oscillator, as recited in claim 8, wherein said material is
aluminum oxide.
10. The oscillator, as recited in claim 7, wherein said means for
stiffening the base is a slab of material having a stiffness
characteristic substantially higher than the stiffness
characteristic of the base portion of said oscillator package.
11. The oscillator, as recited in claim 10, wherein said slab of
material is aluminum oxide.
12. The SAW oscillator, as recited in claim 11, wherein said slab
of material has a thickness in the range of 0.1 to 0.5 inches.
13. The SAW oscillator, as recited in claim 11, wherein said slab
of material has a thickness in the range of 0.3 to 0.5 inches.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to surface acoustic wave
stabilized oscillators and, more particularly, to surface acoustic
wave stabilized oscillators having low vibration sensitivity.
As it is known in the art, surface acoustic wave devices (SAW
devices) are employed in a variety of applications, such as
resonators and delay lines for oscillator circuits, as well as,
filters and pressure transducers. Generally, a SAW device comprises
at least one transducer including a set of conductive members which
is disposed on or recessed within a surface of a piezoelectric
substrate.
In many applications of SAW devices, particularly with respect to
applications of resonators and delay lines, as frequency
stabilizing and determining elements in oscillators, it is
important to provide a package having a relatively small size
while, at the same time, properly mounting the SAW device within
the package to reduce the so-called vibration sensitivity of the
SAW device. It is known that the resonant frequency of an
oscillator, including a SAW device, is sensitive of external
vibration or changes in external stress applied to the SAW device.
The sensitivity results from the external stress on the
piezoelectric substrate causing changes in surface wave velocity
and hence resonant frequency characteristics of the SAW device. In
particular, the surface wave velocity is influenced by applied
forces through the second order elastic coefficients of the
material of the SAW substrate and also because the physical
distance between the transducers or gratings on the substrate is
changed. Thus, both of these factors contribute to net frequency
change in SAW devices such as delay lines and resonators.
Conventional packages such as the TO-8 package and flatpacks which
are hermetically sealed are often used. Such packages are
relatively large in volume in comparison to the size of the SAW
device. These packages also constrained how the SAW substrate can
be mounted within the package to provide a SAW device having a
surface wave velocity which is relatively invariant with external
stress.
It is generally known that a low vibration sensitivity is obtained
when the bottom of the SAW device substrate is uniformally
supported. One approach is to provide a soft stress reducing
material such as a rubber or a room temperature vulcanizing silicon
rubber on the conventional package to uniformally support the
bottom of the SAW device substrate. It has been found, however,
that such a soft material causes long-term frequency shifts in SAW
devices provided within such packages because over time the soft
material will outgas impurities which may become deposited upon the
upper surface of the SAW device substrate. Such deposits of
impurities are believed to cause changes in the velocity
characteristics of the surface waves which propagate along the
upper surface of the substrate and hence change the long term
frequency characteristics of the device. These frequency
characteristics are often of a magnitude so large that the device
is no longer acceptable for many applications. On the other hand,
rigidly fastening the bottom of the SAW substrate to the package is
also generally unacceptable since the thermal expansion
characteristics of the SAW substrate are generally not perfectly
matched to the thermal expansion characteristics of the material of
the package. Because of this mismatch in thermal expansion
characteristics, this arrangement leads to unpredictable
temperature dependent stress characteristics that adversely effect
frequency stability and may even result in fracture of the base of
the SAW device.
Once solution to the foregoing problem has been to provide a
hermetically sealed package in which the SAW substrate provides a
bottom portion of the package. Such arrangements are shown in U.S.
Pat. No. 4,270,105, Parker, et al., issued May 26, 1981, entitled
"Stabilize Surface Wave Device" and assigned to the assignee of the
present invention and in articles entitled "Long Term Aging and
Mechanical Stability of 1.4 Gigahertz SAW Oscillators" by M.
Gliden, et al., Proceedings of the IEEE Ultrasonic Symposium, page
184 and "SAW Resonator Frit Bonded Pressure Transducer" by D.
Weirauch, et al., Proceedings of the IEEE Ultrasonic Symposium,
1979, page 874.
The issued U.S. Patent describes a quartz package having the
hydrophobic polymer coating for passivating an upper surface of the
substrate on which the surface waves propagate. The article by
Gliden describes a SAW based oscillator including a quartz packaged
SAW device. Long term aging data indicates that such devices will
have frequency shifts of at least .+-.4 parts per million per year.
For some applications in stable oscillators, this drift or aging
characteristic is unacceptable. The second article by Weirauch
describes a pressure transducer fabricated having a quartz package.
According to this article, a frit was applied to both surfaces, the
SAW was evacuated, and the substrates were then mated together. As
indicated in the article, a hysteresis effect was observed. This
indicates that there may be a stress relief problem associated with
the technique. It was also suggested that some long term frequency
shift effects may be present. These short term and potential long
term drift problems may be unacceptable for SAW devices such as
delay lines and resonators when used in highly stable precision
oscillators.
A solution to these problems of aging and short term frequency
shift has been the All-Quartz Package (AQP) as mentioned in a paper
entitled "A New All-quartz Package for SAW Devices" by Parker, et
al., 39th Annual Frequency Symposium, 1985, pages 519-524. The
approach has been used to provide devices such as resonators having
very low aging rates (typically less than 0.5 parts per million per
year). The approach has also been used on delay lines. The
distinction made between SAW resonators and SAW delay lines is that
a SAW resonator generally also has a pair of gratings disposed on
the surface wave surface outside of the region of the pair of
transducers. The resonator thus provides a high Q narrow band
device whereas the SAW delay line does not have such a pair of
gratings and is a broaderband, lower Q device.
The vibration sensitivity of SAW devices used in SAW stabilized
oscillators is particularly important for radar applications where
the SAW stabilized oscillator will be in a high vibration
environment, such as, for example, on a plane, helicopter, or
missile. Frequency fluctuations induced by external vibrations can
cause a significant increase in oscillator phase noise levels at
the vibration frequencies and therefore degrade the performance of
the radar. Theoretical predictions of the vibration sensitivity for
the all-quartz package resonators indicate that vibration
sensitivities as low as 10.sup.-11 fractional change in frequency
per g of applied force should be attainable in each of the three
orthogonal directions of the SAW device. Such performance will be
comparable to or better than any known oscillator technology (i.e.
bulkwave technologies or dielectric resonator technologies for
example). In practice, however, it has not been heretofore possible
to provide SAW stabilized oscillations in which the SAW device
exhibits such low SAW vibration sensitivities.
SUMMARY OF THE INVENTION
In accordance with the present invention, a SAW stabilized
oscillator includes means for providing a closed loop having an
integral multiple of 2.pi. radians of phase shift and excess small
signal gain at a desired frequency, said means including a SAW
device having a substrate including a surface which supports
surface wave propagation and a pair of transducers coupled to said
surface wave propagating surface. The SAW oscillator further
includes a package comprising a base portion, said base having a
first stiffness characteristic, and said base supporting said
closed loop providing means. The SAW oscillator further includes
means coupled to said base for increasing the stiffness of said
base. With such an arrangement, the stiffening means reduces
bending of the SAW oscillator package in response to applied
external stresses. By lowering oscillator package bending in
response to external stresses, the SAW device which is supported by
the oscillator package will have concomitantly lower external
bending stresses coupled to it and concomitant therewith lower
vibration sensitivity.
In accordance with a further aspect of the present invention, a SAW
oscillator includes means for providing a closed loop having an
integral multiple of 2.pi. radians of phase shift and excess small
signal gain at a selected frequency. Said means further includes a
surface acoustic wave resonator having a substrate with a surface
which supports surface wave propagation, a pair of transducers
coupled to said surface wave propagation surface, and a pair of
gratings disposed outside of the region occupied by said pair of
transducers for confining surface wave energy to the pair of
transducers. The SAW device further includes a glass frit seal and
a cover used in combination to enclose the surface wave propagation
surface of the SAW substrate. The SAW oscillator further includes
an oscillator package comprising a base having a frame portion
affixed to said base with said base supporting said SAW resonator
and said means for providing a closed loop and said base having a
first stiffness characteristic. A cover is disposed over the base
of the SAW oscillator package and a first slab of a stiff ceramic
material is disposed over the cover to increase the stiffness of
the cover and a second slab of a stiff ceramic material is disposed
over the base to increase the stiffness of the base. With such an
arrangement, a SAW stabilized oscillator having low levels of
vibration sensitivity is provided. The use of slabs of stiff
ceramic material over the cover and base provide an oscillator
package having a relatively small size yet having a high degree of
stiffness which inhibits external stresses from bending the
oscillator package. The reduced bending of the oscillator package
provides concomitantly reduced bending of the SAW resonator. Thus,
providing a SAW stabilized oscillator having low levels of
vibration sensitivity compared to prior techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of this invention, as well as the invention
itself, may be more fully understood from the following detailed
description of the drawings, in which:
FIG. 1 is a block diagram showing the SAW resonator used as a
frequency stabilizing determining element in an oscillator
circuit;
FIG. 2 is a plan view of a SAW oscillator package having a SAW
device and hybrid circuitry used to provide the SAW oscillator
supported in the package;
FIG. 3 is a cross-sectional view taken along lines 3--3 of FIG.
2;
FIGS. 4 and 4A are isometric views showing a SAW oscillator package
having stiffener layers in accordance with the present
invention;
FIG. 5 is a plot of vibration sensitivity for vibrations parallel
to the substrate normal vs. stiffner thickness;
FIG. 6 is a plot of vibration sensitivities in each direction and
total vibration sensitivity vs. vibration frequency using a
standard size SAW device; and
FIG. 7 is a plot of vibration sensitivity in each direction and
total vibration sensitivity vs. vibration frequency using a
miniaturized SAW package.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, a stable oscillator 10 is shown to include
an amplifier 12 disposed in a feedback loop denoted by an arrow 13.
The feedback loop 13 further includes coupling means 14, attenuator
15, input impedance matching network 16, a SAW device 20, an output
impedance matching network 17, and a phase shifter 18. The SAW
device 20, here a resonator, is used in the feedback loop 13 to
stabilize the phase and frequency characteristics of the signal in
the feedback loop 13. The output of the amplifier 12 is coupled to
here the input port of the coupling means 14. The coupling means 14
is here a microstrip type directional coupler although a center tap
transformer may alternatively be used. A first branch output port
of coupler 14 is coupled to the attenuator circuit 15. A second
branch port, here an output port of the coupler means 14, is
coupled to a second amplifier 19. Here, a conventional amplifier
used to provide an amplified output signal to output terminals
11a.sub.1, 11b.sub.1. The coupling means 14 is thus used to divide
the output signal provided from amplifier 12 and to feed a first
portion of the signal to a conventional attenuator and in a second
portion signal to the output amplifier in a predetermined ratio. A
division ratio of 10 to 1 is typically used so that 1 part in 10 of
the signal is fed to the input of the output amplifier and the
remaining portion of the signal is coupled to attenuator 15 and is
fed back to the amplifier 12 via the remaining portion of the
feedback loop 13. The output amplifier 19 is used to provide an
amplified output signal in response to the signal provided by the
oscillator circuit 10 and to feed such amplified output signal to
output terminals 11a.sub.1, 11b.sub.1 and hence to a load 40.
The frequency of the oscillator 10 is related to the closed loop
phase and frequency characteristics of the feedback loop. Such a
circuit will oscillate at a frequency in which the phase shift
around the loop is 2.pi. radians and at which the loop has excess
small signal gain, as is known. The phase and frequency
characteristics of the feedback loop 13 are thus adjusted by the
phase shifter 18 which is used to change the oscillator frequency.
The phase and frequency characteristics of the SAW resonator 20,
phase shift, and other components of the feedback loop, as is known
in the art, also contribute to determining the phase and frequency
characteristics of the loop. The attenuator circuit 15 may
optionally be required to control the signal level in the feedback
loop 13 of the oscillator 10. Also, depending upon the
characteristics of the various circuits, conventional input and
output impedance networks 16 and 17 are optionally used to match
the impedances in the feedback to the impedances of the SAW
resonator 20. Such impedance matching circuits 16, 17 and
attenuator circuit 15 however are not always required.
The SAW resonator 20 or alternatively a SAW delay line (not shown)
in combination with the phase shifter 18 provides a fixed and
coarsely adjustable phase shift to the input of the amplifier 18
thereby supplying the requisite phase shift characteristics to the
input of the amplifier 12 at a particular frequency. Phase shifter
18 could be a coarsely adjustable phase shifter, a electronically
controlled phase shifter provided by varactor diodes, for example,
or other suitable phase shifter elements, as is commonly known in
the art. The major portion of the phase shifter in the feedback
loop, is provided by the resonator 20 (or alternatively delay line)
with the phase shifter 18 providing a relatively small adjustment
in the phase characteristics in the feedback loop thus enabling
tuning of the resonant circuit of the oscillator to a desired
frequency output frequency. Since the SAW device 20 provides a
substantially complete and relatively stable portion of the delay
around the feedback loop, the frequency of the oscillation from the
oscillator will be likewise relatively stable.
In particular, for precision oscillator applications in a vibrating
environment, the effects of applied external stress on the SAW
oscillator results in fluctuations in the frequency of oscillation
of the output signal from the oscillator. It is generally desirable
to reduce such fluctuations to significantly low levels.
Theoretically, oscillators using SAW resonators as stabilizing
elements could exhibit vibration induced frequency fluctuations in
the order of about 10.sup.-11 fractional change in frequency per
"g" of applied force where g is the acceleration due to
gravity.
Referring now to FIG. 2, a typical implementation 10' of the SAW
oscillator 10 (FIG. 1) is shown to include a first dielectric
carrier 21, here comprised of alumina (i.e. aluminum oxide), having
disposed over a first side thereof pattern strip conductors
comprised of here gold (not numbered) and having disposed over a
second surface thereof a ground plane conductor of here gold (not
shown) bonded into an oscillator package 31. The patterned
conductors are arranged to provide interconnections to the
amplifiers 12a', 12b', as shown, as well as the other components of
the oscillator 10'. Here two amplifiers 12a' and 12b' are used to
provide the amplifier 12 (FIG. 1) in the feedback loop 13 to
provide sufficient small signal gain at a desired frequency. The
circuit in FIG. 2 further includes a phase shifter 18', here a
fixed frequency phase shifter comprised of a L-C-L T-network (not
individually referenced), and a directional coupler 14', which with
the amplifiers 12a', 12b' complete the components on carrier 21
which are part of the feedback loop 13 (FIG. 1). Disposed here
outside of the loop is an attenuator 25 as well as the amplifier
19' and a low pass filter 27.
The output from low pass filter 27 is provided to a strip conductor
(not referenced) which is coupled by bond wires to a center
conductor 37a of a standard coaxial connector 37, here an SMA type
connector. Connector 37 is attached to the package 31 via a pair of
screws (not numbered). The package 31 includes a base 33, as well
as, an outer peripheral frame portion 32 which is integrally
provided with the base 33. That is, the base 33 is here machined
from a slab of material (such as "KOVAR" which is gold plated) to
form the recessed base portion and provide the frame 32 disposed
about the periphery of the base 33 of the package 31. A through-pin
36 is disposed through the frame 32 of package 31 and
dielectrically isolated therefrom to provide DC bias to a voltage
regulator integrated circuit on carrier 21. The other circuits on
carrier 31 are connected by conductors from the voltage regulator
29 to provide regulated DC voltages to the various circuits, as
would generally be known to one of skill in the art. Through-pins
38a, 38b are also provided through the package 31 and are used for
testing purposes but are generally disconnected, during user
operation. The strip conductors (not numbered) disposed adjacent
the pins are jumped together with bond wires for normal operation
of the oscillator. Pins 38a, 38b and breaks in the strip conductor
(not numbered) adjacent the pins 38a, 38b are provided such that
during testing and calibration an open loop condition can be
provided which is used to set the various parameters of the loop,
whereas after testing a closed loop is provided by disposing
conductors (not shown) across the break in strip conductors
adjacent the pins and disconnecting the pins. Distributed as needed
on the carrier 21 are ground conductor strips 37 which are coupled
to the ground plane conductor (not shown) on the underside of
carrier 21 by plated vias 37a, as shown.
The oscillator 30 further includes the SAW device here a resonator
20 mounted between a pair of carriers 34a, 34b here also comprised
of alumina and which have on a first underside thereof a ground
plane conductor (not shown) and have disposed on the top surfaces
thereof the ground conductor strips 37 having plated via holes 37a,
as well as, patterned strip conductors used to provide microstrip
transmission lines to interconnect the SAW resonator to the hybrid
circuit generally provided on carrier 21.
The exact implementation of the oscillator circuit generally
denoted as 30 and as described above would be apparent to one of
ordinary skill in the art and any oscillator design would be useful
in practicing the present invention.
Preferably, in order to reduce vibration sensitivity, the
integrated circuits, in particular, the amplifier circuits 12a,
12b, and 19 are bonded to the surface of the carrier 21 by use of
here a non-conductive epoxy. One particular epoxy used is "EPO-TEK"
type H70E-175 (Epoxy Technology, Inc., Billerica, Mass. Conductive
epoxies which are used in the hybrid to fasten components securely
to the carrier include EPO-TEK H20E-175. It is important when
fabricating the particular hybrid circuit 11 on a carrier 21 that
the components be securely affixed to the carrier 21 to prevent
microphonic induced noise in the oscillator.
The SAW resonator is here obtained from Raytheon Company, Research
Division, Lexington, Mass., or Raytheon Microwave and Power Tube
Div Northborough, Mass. (SMDO) and is a resonator, packaged in a
Raytheon All-Quartz Package although another resonator may
alternatively be used. It would be preferred that the base of the
SAW device provide part of the package. In particular, as shown in
FIG. 3, the so-called All-Quartz Package resonator includes a base
20a having transducers (not numbered) disposed in recesses provided
therein and gratings not shown and which is spaced from a cover
portion 20c and secured to the base by a glass frit seal 20b. Here
for best vibration performance the cover 20c portion of the package
is mounted on the base 33 of the oscillator package 31. The cover
20c is thus spaced from the base by the glass frit seal 20c which
in combination provide a sealed enclosure for the resonator. In the
All-Quartz Package the cover and base are crystallographically
matched substrates and are both comprised of an ST-cut of quartz.
The exact ST-cut being specified by the user depending upon
temperature considerations as is known.
The above resonators are generally characterized as having low,
long-term aging drift characteristics on the order of 0.5 parts per
million per year, as well as, low noise and other attributes useful
for high precision fixed frequency oscillator circuits.
As shown in conjunction with FIG. 3, the carriers 34a and 34b are
secured to the base of the package 31 using a conventional epoxy
bond 48 whereas the all-quartz package SAW device 20 is mounted
upside down in the oscillator circuit 33 and is attached to the
oscillator circuit 33 by use of a sheet adhesive 47 generally known
as 3M (Minneapolis, Minn.), "Isotac" A-10 adhesive, part number
Y-9473 having a thickness of 0.010 inches. This adhesive is
preferred although other adhesives or tapes may alternatively be
used.
In general, the oscillator package 31 is comprised of a metal such
as Kovar. The Kovar which is an alloy of 29% Ni, 17% Co, 0.3% Mn
and balance Fe has a certain elastic modulus (Young's modulus)
characteristic. It has a concomitant stiffness characteristic which
is related to the thickness of the base, orientation of the frame,
and elastic modulus of the material of the package 31.
Referring now to FIG. 4, a SAW oscillator having stiffening means
to reduce vibration induced changes in oscillator frequency is
shown to include the packaged device 30 as generally described in
conjunction with FIG. 2 having disposed thereover a cover 39 here
also comprised of gold plated Kovar as for package 31 and which is
seam welded to the package 31 using conventional seam welding
techniques known in the art. Disposed on the bottom portion of
package 31 is a SAW package stiffening member 42 here a slab
comprised of a high Young's modulus material which exhibits a high
degree of stiffness (that is, rigidity) and which is used to
stiffen or make more rigid the base 33 of the oscillator package 31
and thus couple a reduced magnitude of external stresses to the SAW
resonator.
As with the stiffness or rigidity characteristic of the package,
the stiffness of the slabs is related to the thickness of the slab
42 and the modulus of elasticity of the material of the slab. Thus,
the thicker the slab, the higher the stiffness. For most practical
applications, particularly where space considerations are a factor,
slabs having a thickness in the range of about 0.1 inches to 0.5
inches are preferred. A preferred range is 0.3 inches to 0.5
inches. Greater thickness could, of course, be used if space and,
in particular, height are not restricted. The stiffening member 42
is here a slab of aluminum oxide. Disposed over cover 39 is a cover
stiffening member 44 here comprised of a material having a high
degree of stiffness and particularly a high Young's modulus. The
material here used is also aluminum oxide since aluminum oxide has
good thermal match properties to the Kovar. The cover stiffner
member 42 has a thickness in the range of 0.1 inches to 0.5 inches
also. Other materials having higher Young's modulus and thus higher
degrees of stiffness may alternatively be used for both cover
stiffener 44 and base stiffener 42.
The oscillator package 31 is firmly affixed between the base
stiffener 42 and cover stiffener 44 by use of a thermoplastic
sheets 43, 45, respectively, which soften at an elevated
temperature and then provide bonds between the package 31 and the
respective stiffener members 42 and 44 to secure such members to
the package 31. A preferred material used to fasten the package 31
to the stiffener members is "Chomerics Chobond" type 1670
(Chomerics, Inc., Woburn, Mass.). Other adhesive fasteners may
alternatively be used. Here the thermoplastic sheet is used to
permit reworking of the devices if necessary since the stiffener
members can be relatively easily removed from the base 33 and cover
39 portions of the package 31. Alternative materials can also be
used to fasten the stiffeners to the packaging cover. For example,
Crystal-bond 555 from Aremco, Inc. (Ossining, N.Y.), a waxy type of
bond, as well as, Crystal-bond 509 from Aremco which is also a waxy
type of bonding adhesive may alternatively be used. It is preferred
to have a somewhat elastomer type of bonding adhesive between the
Kovar and the alumina stiffness to take into consideration the
small, but nevertheless, important differences in coefficients of
thermal expansion between the two materials if the oscillator 40 is
to be operated over varying temperature ranges.
Low vibration sensitivity is provided from the device as described
in conjunction with FIGS. 1-4 by providing a SAW resonator having a
glass frit geometry suitable for low vibration sensitivity, as
generally described in an article entitled: "An Analysis of the
Normal Acceleration Sensitivity of ST Cut Quartz Surface Wave
Resonators Rigidly Supported along the Edges" by Tiersten, et al.,
Proceedings of the 41st Annual Symposium on Frequency Control,
1987, pages 282-288. Any one of the preferred frit geometries
identified by the authors in the paper may be used to provide
optimum performance. The theory set forth by the authors state that
with proper choice of the frit geometry, vibration sensitivities in
the low 10.sup.-11 fractional parts frequency per "g" of applied
stress range would be obtainable, such low vibration sensitivity
levels have not been observed in practice. One of the problems with
the theoretical analysis is that it is based on the assumption that
the frit geometry is perfect everywhere and that there are no
misalignment or local deviations from straight lines. Furthermore,
another assumption is that the applied acceleration is uniformally
distributed over the entire mounting surface of the SAW device. In
practice neither one of these assumption is perfectly true. That
is, the SAW device is generally not mounted uniformally over the
Kovar package base and due to differences in the melt
characteristics of the glass frit, the glass frit pattern will
deviate from the ideal theoretical patterns described in the above
article.
In accordance with this invention, the SAW oscillator package 31 is
stiffened to provide a reduction in vibration sensitivity. In
response to external stresses on the package 31, strains are
provided in the material of the package. That is, the package 31
bends in response to applied external stresses. Bending and
straining of the package causes concomitant bending and straining
of the SAW resonator 20 which is mounted to the package base 33
(even though the SAW resonator may be mounted upside down on the
package). At the vibration levels of interest (DC to 10's of KHz),
such minute strains and bending become extremely important.
In accordance with the invention, by stiffening the Kovar package,
that is, by providing a slab of very stiff material on the base
portion of the package, as well as, a slab of very stiff material
over the cover portion of the package, the amount of bending and
flexing of the Kovar package in response to applied forces is
significantly reduced and thus concomitant therewith the bending of
the SAW oscillator is also significantly reduced.
As shown in FIG. 5, the vibration sensitivity of an oscillator
package as generally described in conjunction with FIGS. 1-4 for a
vibration frequency of 1.5 kilohertz is shown as a function of
various thicknesses of an alumina stiffener. The point 51 shows the
vibration sensitivity is very large when the alumina thickness is
extremely small (i.e. here less than 0.1 inches) whereas the
vibration sensitivity drops as a function of increasing thickness
of the package stiffener 42. The last data point 55 corresponds to
a stiffener of 0.48 inches thickness firmly mounted on a 2.5 inch
cube of aluminum and provides oscillators having a vibration
sensitivity of approximately 9.5.times.10.sup.-11. From this data
it can be deduced that for most applications a base stiffener
having a thickness between approximately 0.3 and 0.5 inches would
be a reasonable compromise between size and performance.
It has also been found that to minimize the problem of non-uniform
distribution of acceleration across the SAW resonator and to thus
minimize the effects of less than ideal frit geometries, the
rigidity of the SAW device itself should also be increased. This
can be accomplished by either reducing the lateral dimensions of
the substrate or increasing the thicknesses of the substrate or
both. Reduced lateral dimensions not only significantly increases
rigidity but also decrease the surface area coupled to external
bending forces.
FIG. 6 shows a typical vibration sensitivity for all three axis of
a SAW oscillator that uses an All-Quartz Package SAW resonator
(Raytheon Company, Research Division) measuring 0.4 by 0.5 inches,
a commonly used size. The thickness of the SAW substrate and cover
are both 0.035 inches. The FIG. depicts vibration sensitivities
where .gamma..sub.1 is the vibration sensitivity for vibrations
parallel to the substrate normal (curve 61), .gamma..sub.2 is the
vibration sensitivity for vibrations perpendicular to the acoustic
propagation direction in the plane of the substrate (curve 63),
.gamma..sub.3 is the vibration sensitivity for vibrations parallel
to the acoustic propagation direction in the plane of the substrate
(curve 65) and .GAMMA..sub.3 is the vector sum of the sensitivities
for the three perpendicular directions (curve 6). As can be seen,
the vector sum of the individual vibration sensitivities
(.GAMMA..sub.3) as a function of vibration frequency generally lie
in the range of 1.times.10.sup.-10 to 5.times.10.sup.-10
(g.sup.-1).
FIG. 7 shows measured vibration sensitivity for a SAW oscillator
having an All-Quartz Package SAW resonator (Raytheon SMDO) that
measures 0.16 by 0.33 inches. The substrate and the cover of the
SAW package again were 0.035 inches thick. Even though the frit
geometry used in the SAW package is not one of the ones identified
in the article as being optimum, the overall vibration sensitivity
for the device measured in FIG. 7 is significantly lower than that
shown in FIG. 6. These characteristics for both devices were
measured using conventional SAW oscillator packaging techniques.
Unlike the SAW oscillator depicted in FIG. 2, the resonator used in
FIG. 7 corresponds to a "Mini-AQP" package and is provided by
disposing the glass frit as close to the SAW resonators as is
practical allowing for slight glass reflow during the manufacturing
operation of providing the glass frit seal. The aspect ratio of the
glass frit seal is 2.2. This permits the SAW package for the
resonator to be substantially smaller than the conventional SAW
All-Quartz Package described in conjunction with FIG. 2. With a
smaller package, the SAW device itself is also more rigid and thus
has a lower vibration sensitivity. Further, with a smaller package
there is also a smaller surface area through which to couple
applied external stresses.
A preferred technique to fabricate an All Quartz Package is
generally described in copending applications Ser. No. 221,449
filed Jun. 29, 1988 by Greer et. al. and Ser. No. 650,017 filed
Sep. 13, 1984 by Borcheldt et. al. both assigned to the assignee of
the present invention and both incorporated herein by reference. To
provide a mini-AQP, the glass frit seal is positioned as close as
possible to the interdigitated transducers without touching the
transducers or interfering with surface wave propagation. The cover
and base substrates are also cut correspondingly as small as
possible to thus reduce the overall size of the package.
Optimally, a "mini-AQP" may be used in an oscillator package 31
having stiffeners 42, 44 as described in conjunction with FIGS.
2-4.
In high vibration environments, it is desirable to mount the SAW
oscillator packages on a vibration isolation mount as would be
known by one of skill in the art.
One example of an isolation mount which would be preferred would
include a mounting plate, comprised of a stiff metal, having four
holes disposed on thinned corner regions of the plate to receive
plastic or rubber type grommets. A preferred grommet is a type
obtained from EAR, Inc., EAR Type 401-1 fabricated from
polyvinylchloride (PVC). Generally, such an assembly would be
desired to have a known fundamental resonant frequency. To insure
optimum operation and hence maximum damping of vibrations, the
grommets which serve as mounting holes for the plate are
prestressed by passing fasteners through each hole in the grommet,
and the grommets as well as the rest of the assembly are maintained
at a predetermined temperature as here 60.degree. C. This maintains
the proper temperature for the SAW device and proper durometer
rating for the grommets.
Having described preferred embodiments of the invention, it will
now become apparent to one of skill in the art that other
embodiments incorporating their concepts may be used. It is felt,
therefore, that these embodiments should not be limited to
disclosed embodiments, but rather should be limited only by the
spirit and scope of the appended claims.
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